Manufacturing formulations involving aqueous solid dispersions often require to be processed at a high concentration of solids to reduce costs of transportation, to increase laydown and to reduce the drying load, as the removal of water is wasteful and expensive. The formulations often also need to be stable over a wide range of shear, which extends from low to high shear rates or stresses. After periods of high shear, aqueous dispersions of boehmite, especially those containing high concentrations of solid, tend to possess either a high viscosity at low shear or a tendency to form a gel or a yield stress material, depending on the rate of decrease from high shear. The high viscosity of the dispersions at low shear affects their dispersability and flowability, making the systems difficult to stir, pump, transport, coat or pour. Gelation is even more of a problem because it is often irreversible and consequently totally unacceptable for manufacturing processes that rely on systems to flow.
Dispersible boehmite alumina powders have traditionally been used in applications such as sol-gel ceramics, catalysis, refractory materials, rheology control and surface-frictionising. Other more recently developed uses include surface coating as well as paint detackyfication.
A composition containing boehmite is particularly useful in the fabrication of coated inkjet media for inkjet printing, insofar as the dry-coated system is both porous and glossy. The porous nature of the media is useful for the rapid drying of the inks after printing to enable earlier customer handling and to minimize the mottle or coalescence defect: the glossy nature is popular with customers who want rich photographic images. Unfortunately gloss and porosity do not often go hand-in-hand and hence formulations such as those containing boehmite that deliver both are relatively rare and very sought after. However the high viscosity at low shear and/or the tendency to form a gel or a yield stress material in practice limits the use of boehmite in such systems.
It is known in the art that an inorganic, organic or polymeric form of an acid may be added to an aqueous slurry of alumina. Thus U.S. Pat. No. 3,935,023 describes the use of hydrochloric acid to form stable dispersions of alumina, whilst EP-A-0 736 491 teaches the use of a viscosity-reducing agent for an alumina sol based on a compound having a carboxyl group or a sulfonic acid group or a compound selected from the group consisting of formamide or acetamide, relating to small non-polymeric molecules. U.S. Pat. No. 6,171,573 discloses a process for producing alumina sols using an inorganic monobasic acid, such as hydrofluoric acid, or a lower aliphatic monobasic acid, such as formic acid, and U.S. Pat. No. 6,476,083 teaches an alumina dispersant comprising a monocarboxylic acid containing at least one nitrogen atom in its molecule.
U.S. Pat. No. 6,264,710 describes the use of monoprotic acids, such as nitric acid, as peptising agents or dispersion aids for producing more stable alumina (preferably boehmite) sols and suggests that multiprotic acids be avoided since they can rapidly gel the dispersion or sol, making it difficult to handle or to mix in additional components. U.S. Patent Application No. 2002/0027304 discloses the use of acetic acid, citric acid or a polyacrylate as a viscosity modifier for alumina-boehmite systems.
There are a number of publications which further teach the use of acids as dispersion stabilizers for alumina suspensions, such as, for example, ‘Ceramic Development: Programmne Research at the Swedish Ceramic Institute 1993-1996’ and Journal of Ceramic Processing Research, 3 (2002) 10-14 ‘Suspension Systems for Coagulation Processing’ by C. Pagnoux, which discloses the use of a low molecular weight sulfonic acid (TIRON) and a polyacrylic acid (DISPEX N40).
P. C. Hidber et al., Journal of the European Ceramic Society, 17 (1997) 239-249 ‘Influence of the Dispersant Structure on Properties of Electrostatically Stabilized Aqueous Alumina Suspensions’ describes a dispersant based on a hydroxy- and carboxy acid-substituted benzene ring whilst A. U. Khan et al., J. Mater. Chem., 12 (2002) 1743-1747 ‘The Strength of Colloidal Interactions in the Presence of Ceramic Dispersants and Binders’ discusses the use of the ammonium salt of aurintricarboxylic acid (ALUMINON) as a dispersant of alumina dispersions. The use of citric acid as an alumina dispersant is disclosed in A. R. Studart et al., Journal of the European Ceramic Society, 23 (2003) 997-1004 ‘Selection of Dispersants for High-Alumina Zero-Cement Refractory Castables’ and also in P. C. Hidber et al., J. Am. Ceram. Soc., 79[7] (1996) 1857-67 ‘Citric Acid—A Dispersant for Aqueous Alumina Suspensions’.
In addition to the use of an acid as an alumina dispersant, the use of an acrylamide polymer for flocculating boehmite is disclosed in U.S. Pat. No. 3,117,944.